Opioid receptor agonists are highly effective at producing analgesia, but their clinical use is hindered by the development of tolerance and high abuse liability resulting from activation of the neural reward circuitry. Therefore, understanding how opioids regulate reward circuits and how opioid receptor signaling changes with opioid exposure are key steps towards overcoming current clinical limitations of opioid therapy. One important reward pathway that releases and responds to opioids includes the hypothalamic proopiomelanocortin (POMC) neurons. Recent data show that POMC neurons are regulated by opioids acting on POMC neurons and also by opioids acting on terminals presynaptic to POMC neurons. Interestingly, while postsynaptic mu opioid receptors on POMC neurons undergo desensitization within minutes, presynaptic mu opioid receptors that mediate the inhibition of transmitter release do not readily desensitize. The goals of the present proposal are to 1) determine the mechanisms underlying the presynaptic resistance to desensitization and 2) to determine the consequence of differential pre- and postsynaptic opioid receptor regulation. These goals will be achieved using electrophysiologic, optogenetic and imaging approaches in brain slices. The results may provide insight into ways to prevent postsynaptic receptor desensitization and will thoroughly define how opioids affect neuron activity in a reward circuit via pre- and postsynaptic receptors during acute and chronic exposure to opioids.